The photocatalytic activity of nano-Bi2O3 was evaluated in degradation of rhodamine B (RhB) as a model of dye pollutant from waste waters. Nano sized Bi2O3 was synthesized using the chemical precipitation method. The as-prepared sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FT-IR). Structural analysis revealed that Bi2O3 contains a unique well-crystallized phase and the average crystallite size of 22.4 nm. The SEM analysis revealed that the size of Bi2O3 particles was mainly in the range of 16-22 nm. Response surface methodology was applied to design experiments and to optimize the photocatalytic process. A second order model was developed and a good correlation was found between experimental and predicted responses, confirming the reliability of the model. The optimal condition for maximum degradation of values of RhB resulted in initial concentration, irradiation time, initial pH and catalyst dosage of 12.5 mg.L-1, 120 min, 4.6-7 and 0.75 g.L-1, respectively. The Bi2O3 nanoparticles exhibited an efficient ultraviolet photocatalytic activity so that under optimal condition more than 95% of Rhodamine B was decolorized.The Pareto analysis indicated that the order of relative importance of the input variables on the dye degradation efficiency is as follows: Bi2O3 dosage>pH ≈ irradiation time>initial concentration of the RhB.